The present invention relates to a method of producing three dimensional skeletal structures of continuous interconnected strands, the structures having a large void volume and wherein the void volume is substantially interconnected throughout the skeletal structure. More particularly, the present invention relates to the preparation of this skeletal structure using deformed and usually subsequently removable beads which form a mold.
The preparation of similar forms of skeletal structures from polymeric materials is known to the prior art. Thus the preparation of foamed or cellular isocyanate derived polymers is well known. In general, the foamed polyurethanes have a three dimensional skeletal structure of interconnected strands and in addition have membranes joined to the strands which divide contiguous cells.
To foam the isocyanate derived polymers, a gas bubble is generated in situ in the liquid polymer material which expands such that contiguous cells are formed. The strands forming the cells generally outline the edges of a polyhedron with the membranes forming the faces of the polyhedron. The membranes can be continuous between the strands or cracked or broken to form an interconnection between adjacent cells, but in either case the membranes restrict fluid flow between adjacent cells.
In recent years there has been considerable effort expended in developing commercially acceptable processes for the removal of the membranes from foamed polymeric materials, particularly isocyanate derived polymers. Thus thermal processes wherein the membranes are selectively destroyed have been developed such as described in U.S. Pat. Nos. 3,175,025; 3,175,030 and 3,329,759 and chemical processes such as described in U.S. Pat. No. 3,171,820 wherein the membranes are hydrolytically destroyed.
Processes have been described for foaming materials besides isocyanate derived polymers such as vinyl and rubber materials but difficulties have been encountered in providing an interconnected void space as well as a relatively uniform cell size. The foaming of metals has been described in U.S. Pat. Nos. 2,937,938 and 2,974,034. Where such foaming operations can be used, membranes dividing contiguous cells are produced.
The preparation of porous structures using various molding techniques is well known to the prior art. Thus British Pat. No. 853,030; U.S. Pat. Nos. 2,721,378; 2,805,208; 2,838,829; 3,202,733 and an article by P. Schwartzkopf from the International Journal of Power Metallurgy 2(4) (1966) pages 3 to 11. In each instance the structures formed have relatively very low porosities in the range of about 30% and to a maximum of about 70%. These porous structures are practically closed cell. In general, the upper limit of porosity and the interconnection of the voids has been dictated by the mold structure used.
It is therefore an object of the present invention to provide a method for the preparation of three dimensional skeletal structures which does not involve foaming or the production of membranes. It is further an object of the present invention to provide a method for the preparation of such skeletal structures which is simple and economical. These and other objects will become increasingly apparent by reference to the following description and the drawing.